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Stock-based Compensation Plans (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
May 20, 2014
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Share Based Compensation Arrangement by Share Based Payment Award, Reduction in Shares Available for Grant Per Share Issued 3.8        
Stock-based compensation expense   $ 20.1 $ 17.4 $ 14.9  
Tax benefit from compensation expense   $ 4.2 3.7 3.1  
Vesting period   3 years      
Award expiration Period   10 years      
Cash received for options exercised   $ 6.3 14.4 14.8  
Share-based Compensation Arrangement by Share-Based Payment Award, Options, Tax Benefit Realized from Options Exercised   2.1 $ 1.7 $ 1.6  
Aggregate intrinsic value, outstanding   29.9      
Aggregate intrinsic value, exercisable   $ 29.1      
Weighted average remaining contractual life, outstanding (in years)   6 years 7 months 6 days      
Weighted average remaining contractual life, exercisable (in years)   5 years 8 months 12 days      
Weighted average grant date fair value of options granted   $ 14.45 $ 18.12 $ 16.30  
Fair value of vested shares   $ 4.6 $ 9.5    
Vesting Each Year [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Vesting rate   33.33% 33.33% 33.33%  
ESPP Plan          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Number of shares authorized under plan 2,500,000        
Number of shares available for grant   2,332,094      
Option or SAR [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Share Based Compensation Arrangement by Share Based Payment Award, Reduction in Shares Available for Grant Per Share Issued 1        
Employee Stock Option [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Unrecognized compensation expense   $ 2.6      
Unrecognized compensation expense, weighted average period of recognition   1 year 4 months 24 days      
Market-Based Restricted Stock Units [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Vesting period   3 years 3 years 3 years  
Granted, Shares   21,379 24,410 31,866  
Forfeited, Shares   1,949 1,674 10,330  
Market-Based Restricted Stock Units [Member] | 2017 Grants [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Granted, Shares   13,532 5,820    
Increase as grant in period, percentage   100.00%   100.00%  
Granted, Weighted Average Grant Date Fair Value   $ 80.92 $ 75.95    
Market-Based Restricted Stock Units [Member] | 2016 Grants [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Granted, Shares     5,820    
Increase as grant in period, percentage   100.00%      
Granted, Weighted Average Grant Date Fair Value     $ 75.95    
Market-Based Restricted Stock Units [Member] | 2015 Grants [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Granted, Shares         3,115
Increase as grant in period, percentage     100.00%    
Granted, Weighted Average Grant Date Fair Value       $ 70.24  
Time-based Restricted Stock Units [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Vesting period   3 years 3 years 3 years  
Granted, Shares   143,128 149,698 152,529  
Granted, Weighted Average Grant Date Fair Value   $ 117.72 $ 117.60 $ 111.66  
Forfeited, Shares [1]   32,821 37,520 45,866  
Performance and Market-Based Restricted Stock Units [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Increase in intrinsic value of awards vested   $ 2.0 $ 1.1 $ 0.6  
Aggregate intrinsic value   $ 11.6      
Weighted average remaining contractual life   1 year 3 months 18 days      
Performance and Market-Based Restricted Stock Units [Member] | Minimum [Member] | Senior management [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Performance metric, potential range   0.00%      
Performance and Market-Based Restricted Stock Units [Member] | Maximum [Member] | Senior management [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Performance metric, potential range   150.00%      
Performance-based restricted stock units [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Granted, Shares   19,504 18,195    
Forfeited, Shares   1,074 996 43,449  
Performance-based restricted stock units [Member] | Minimum [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Performance metric, potential range     0.00%    
Performance-based restricted stock units [Member] | Maximum [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Performance metric, potential range     150.00%    
Restricted Stock and Restricted Stock Units [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Increase in intrinsic value of awards vested   $ 4.5 $ 3.7 $ 2.9  
Restricted Stock Units [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Aggregate intrinsic value   $ 40.8      
Weighted average remaining contractual life   1 year 3 months 18 days      
Unvested Restricted Stock Units and Performance and Market-Based Restricted Stock Units [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Unrecognized compensation expense   $ 25.1      
Unrecognized compensation expense, weighted average period of recognition   1 year 9 months 18 days      
2014 Stock Plan [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Number of shares authorized under plan 6,100,000        
Number of shares available for grant   2,790,071      
Cash received for options exercised   $ 3.9 12.3 14.3  
Intrinsic value of options exercised   $ 2.7 $ 10.1 $ 9.6  
[1] As a result of the sale of Chaucer and included in forfeitures in 2018 are 19,655 shares of time-based restricted stock units, 43,449 shares of performance-based restricted stock units and 2,705 shares of market-based restricted stock units that were awarded to Chaucer employees.